1. Field of the Invention
The present invention relates to optical deflection devices and optical deflection methods.
2. Description of the Related Art
Conventionally, known optical deflection devices deflecting and emitting incident light by using optical deflection elements include electro-optical devices that use materials having great primary electro-optical effect (Pockels effect), such as KH2PO4 (KDP), NH4H2PO4 (ADP), LiNbO3, LiTaO3, GaAs and CdTe, and materials having great secondary electro-optical effect, such as KTN, SrTiO3, CS2 and nitrobenzene, and include acousto-optical devices that use materials such as glass, silica and TeO2 (for example, refer to “Optoelectronic Device” edited by Masaharu Aoki, Shokodo). Also, there are proposed various optical deflection devices that use optical deflection elements including liquid crystal materials.
For example, as disclosed in Japanese Laid-Open Patent Applications No. 6-18940 and No. 5-313116, there are optical deflection devices (optical beam shifters) that reduce light losses. In addition, as disclosed in Japanese Laid-Open Patent Applications No. 2000-193925, No. 9-133931, and No. 5-204001, there are noptical deflection devices that are configured to reduce power of an optical deflecting operation by optical deflection elements and to achieve a smaller size.
Further, as disclosed in Japanese Laid-Open Patent Applications No. 6-194695, No. 6-258646, and No. 6-222368, there are optical deflection devices that extend deflection angles of optical deflection elements. Additionally, as disclosed in Japanese Laid-Open Patent Applications No. 9-133904, No. 2000-507005 (corresponding to International Publication No. WO98/30934), and No. 11-109304, there are optical deflection devices capable of adjusting deflection angles of optical deflection elements. Some of such optical deflection devices can adjust deflection angles of light paths of optical deflection elements without using mechanical moving parts that make the construction complicated, as disclosed in Japanese Laid-Open Patent Applications No. 7-64123 and No. 8-262391.
However, in a case where the electro-optical devices that use the materials having great primary electro-optical effect (Pockels effect) and materials having great secondary electro-optical effect, acousto-optical devices, and the like are used as optical deflection elements, generally, it is necessary to make the light path lengths long so as to obtain sufficiently great optical deflection amounts. For this reason, the present situation is that it is difficult to achieve smaller optical deflection devices, and the use of optical deflection devices is limited since the materials are expensive.
By the way, the above-described optical deflection devices are used for projection optical systems of image display apparatuses that display images displayed on image display elements on such as screens by using the projection optical systems, and for optical switches that use light path shift of an outgoing light with respect to an incident light.
Some image display apparatuses using optical deflection devices display images with improved apparent resolutions by shifting images displayed on image display elements at high-speed in accordance with time by optical deflection elements so as to cause afterimage phenomena in visual perception of human beings. The timings (shift timings) of optical deflection operations by optical deflection devices used for such image display apparatuses must be at speeds high enough to cause afterimage phenomena to visual perception of human beings and must not cause blurring in each image.
However, with the technique disclosed in Japanese Laid-Open Patent Application No. 6-18940, for example, it is difficult to make the speed of response faster to the order of sub-milliseconds, since a nematic liquid crystal is used for the liquid crystal material. Additionally, in the technique disclosed in Japanese Laid-Open Patent Application No. 9-133904, a smectic-A ferroelectric liquid crystal is used for the liquid crystal material. However, since a liquid crystal material in a smectic A phase does not produce spontaneous polarization, it is difficult to make the speed of response high enough. As described above, the optical deflection devices aimed at simplifying the constructions and miniaturization have problems in that it is difficult to speed up the light path shift operations because of the characteristics of the liquid crystal materials used.
Further, as disclosed in Japanese Laid-Open Patent Application No. 5-313116, for example, in a case where the light path shift operation is performed by moving each member arranged on a light path, it is necessary to move in parallel each member arranged on the light path at a high speed and with accuracy, which requires precision and durability of moving parts. With the above-described technique, light losses can be reduced, but problems arise such as occurrence of vibration and noise, and increase in the size of the apparatus.
In addition, as disclosed in Japanese Laid-Open Patent Application No. 6-324320, for example, there is disclosed an image display apparatus that divides an image displayed on an image display element into a plurality of fields, displays an image for each of the fields, and shifts the light path of each of the corresponding fields.
However, with the technique described in Japanese Laid-Open Patent Application No. 6-324320, the construction for driving the optical deflection element becomes complicated, which leads to high cost.
Additionally, Japanese Laid-Open Patent Application No. 10-133135, for example, discloses a technique aimed at entire miniaturization and achieving high precision and high resolution by interposing a translucent piezoelectric element between transparent electrodes and applying voltage so as to vary the thickness and shift the light path.
However, the technique disclosed in Japanese Laid-Open Patent Application No. 10-133135 requires a comparatively large transparent piezoelectric element, and thus the cost of the apparatus is increased.
As described above, in the conventional techniques, it is impossible for the optical deflection devices aimed for simplifying the constructions and miniaturization to sufficiently speed up the light path shift operations. Also, the optical deflection devices intended to speed up the light path shift operations have problems such as complexity of the constructions of the apparatuses, increase in cost caused by the complexity of the constructions of the apparatus, and increase in the sizes of the apparatuses.
The inventors of the present invention have found that high-speed pixel shift can be achieved with a comparatively simple construction: an optical deflection element that performs pixel shift such that liquid crystal molecules are oriented substantially perpendicularly between a pair of boards, and an electric field is generated in a direction substantially parallel to a liquid crystal layer so as to vary the direction of the liquid crystal molecules in a desired direction.
With the optical deflection element, by applying an ac voltage (for example, a square-wave voltage) on the order of several hundred Hz between the pair of electrodes, it is possible to emit lights by switching the light path of incident light in two directions with switching timing of several hundred Hz. As described above, the light path shift uses afterimage phenomena of eyes of human beings. Hence, the switching timing of the light path of incident light may be equal to or more than 30 Hz. However, in order to positively prevent flicker, preferably the switching timing is set to a hundred to several hundred Hz.
By the way, in such an optical deflection element, there is a case where a liquid crystal part becomes clouded when creating the optical deflection element or with successive light path shift driving. In a case where liquid crystal molecules are uniformly oriented perpendicularly in a liquid crystal layer, a black cross-like conoscope image, which is called isogyre, can be clearly observed in the liquid crystal layer. In the clouded part (white turbidity), the conoscope image is very indistinct. Additionally, isogyre is not observed at all in the strongly clouded part. This is the evidence that the perpendicular orientation state of the liquid crystal molecules is disturbed. The director of the liquid crystal molecules in the clouded part is irregular, and it is impossible to obtain a good light path shift function with the clouded optical deflection element. Such white turbidity may occur due to such as influence of an external electric field while the operation of the optical deflection element is suspended for a long time or even for a short period of time. In addition, with the optical deflection element where liquid crystal molecules being greatly disturbed though white turbidity does not occur, there is fear that reliability is degraded.
By the way, it is conceived that disturbance of orientation of liquid crystal molecules and white turbidity may occur due to influence of an external electric field and temperature variation while suspending the operation of optical deflection element. The optical deflection operation with white turbidity remaining causes light scattering and reduction of reliability through growth of alignment defect. Actually, it is confirmed that good light path shift is obtained by bringing liquid crystal molecules to the perpendicular orientation state before starting the optical deflection operation, even if there is no defect such as white turbidity.